The broad goal of this renewal proposal is to define the basic principles of CFTR channel gating and how the most common CF mutations disrupt this process. Like other ABC transporters, CFTR has two nucleotide binding domains (NBDs) that bind ATP in pockets at the interface of an NBD dimer. CFTR also has a unique regulatory domain (R domain) that inhibits channel opening unless phosphorylated by protein kinase A (PKA). Outstanding issues include the degree of coupling between ATP binding and channel opening and how R domain phosphorylation stimulates channel activity. During the current funding period we made several significant discoveries that shed light on the CFTR gating mechanism; notably; (i) certain point mutations in the cytosolic loops that connect the NBDs to the pore promote constitutive (ATP-independent) channel opening; (ii) these mutations rescue the defective gating of a common CF mutant channel (G551D); (iii) CFTR gating is well described by an allosteric mechanism in which ATP binding shifts the equilibrium between pre-existing closed and open states and (iv) the R domain regulates channel activity independent of either ATP binding or NBD dimerization. These findings set the stage for the next phase of our project in which we will pursue three specific aims. Aim 1: Test a model in which the cytosolic loops function as a compression spring that strongly resists unliganded CFTR channel opening, and determine whether our CFTR findings can be generalized to a related ABC transporter (yeast YOR1 exporter). Aim 2: Test predictions of an allosteric gating model that address the link between channel opening and nucleotide occupancy. Aim 3: Define the link between R domain phosphorylation and CFTR channel gating. This project should significantly improve our understanding of the basic principles of CFTR channel gating, and may lead to new approaches for treating CFTR-related diseases.